1. Field
The present disclosure relates generally to reducing effects of atmospheric electrostatic discharges on aircraft and, in particular, to a method and apparatus for reducing atmospheric electrostatic discharges on composite structures on an aircraft.
2. Background
Aircraft are now being designed and manufactured with greater and greater percentages of composite materials. Some aircraft may have more than 50 percent of their primary structure made from composite materials. Composite materials are used in aircraft to decrease the weight of the aircraft. The decreased weight improves performance features, such as payload capacities and fuel efficiency. Further, composite materials also provide longer service life for various components in the aircraft.
Composite materials are tough, light-weight materials created by combining two or more components. For example, a composite material may comprise fibers and resins that are combined and cured to form the composite material.
Carbon fiber reinforced plastic (CFRP) is an example of a composite material that is increasingly used for structural components in commercial aircraft in place of traditional aluminum structures. Carbon fiber reinforced plastic is one type of composite material that may be used in the skin, spars, and ribs of an aircraft. Carbon fiber reinforced plastic is about 2,000 times more resistive than most metals. These types of composite materials are used, because these materials provide a higher strength-to-weight ratio than aluminum.
These types of composite materials are more resistive than the aluminum that they replace. Atmospheric electrostatic discharges on aluminum typically do not result in inconsistencies that may affect the desired performance of different structures in the aircraft. These atmospheric electrostatic discharges may include lightning. Aluminum is more effective at conducting and dispersing currents from these discharges away from a point of attachment.
In contrast, composite materials often act as a dielectric. As a result, composite materials may react differently to atmospheric electrostatic discharges that contact an aircraft as compared to aluminum.
For example, inconsistencies may occur at the point of entry, exit, or both for the atmospheric electrostatic discharge. For example, an atmospheric electrostatic discharge may cause arcing between a fastener and a hole in the structure in which the fasteners are used to hold the different structural components of the aircraft together.
With less conductivity than aluminum, carbon fiber reinforced plastic is more prone to breakdown when subjected to currents from atmospheric electrostatic discharges as compared to aluminum. This type of arcing may induce an inconsistency on the surface of the aircraft. This inconsistency may take the form of vaporization of material or even fracture and through penetration.
With composite materials in aircraft, the creation of inconsistencies from currents caused by atmospheric electrostatic discharges may be reduced through the use of discharge protection systems. These types of systems may result in a reduction of undesired fracture, arcing at structural joints, fuel couplings, hydraulic couplings within a fuel tank, and other components.
Discharge protection systems for composite structures may be attached to or integrated as part of the skin of an aircraft. Protection against atmospheric electrostatic discharges may be used not only for the composite skins and underlying structures, but also for protection against attachment to underlying systems.
Although effective, the currently-used systems are complex and may increase the costs of an aircraft more than desired. For example, the design of an atmospheric electrostatic discharge protection system may take more time and expense as compared to systems that use aluminum structures. Further, these discharge protection systems for composite structures may affect the design of other systems in the aircraft. As a result, the use of protection systems against atmospheric electrostatic discharges may require changes to provide a desired level of protection against these types of events.
Therefore, it would be advantageous to have a method and apparatus that takes into account at least some of the issues discussed above as well as possibly other issues.